It has been known for some time that the surface composition of a metal alloy may have a significant impact on its utility. It has been known to treat steel to produce an iron oxide layer that is easily removed. It has also been known to treat steel to enhance its wear resistance. The use of stainless steels has heretofore relied upon the protection (e.g. against corrosion and other forms of material degradation) afforded by a chromia surface. As far as Applicants are aware there is not a significant amount of art on treating steels to significantly reduce coking in hydrocarbon processing. There is even less art on the types of surface that reduce coking significantly in hydrocarbon processing.
There has been experimental work related to the nuclear industry that spinels similar to those of the present invention can be generated as outer stainless steel surfaces. However, these spinels are thermo-mechanically unstable and tend to delaminate. This is a limitation which tends to teach against using such surfaces commercially. These surfaces have been evaluated for use in the nuclear industry but to Applicants' knowledge have never been commercially used.
In the petrochemical industry due to its thermo-mechanical limitations spinels similar to those used in the present invention are believed to be overall less protective than chromia. It is also believed from a coke make perspective spinels similar to those used in the present invention are not considered to be more catalytically inert than chromia. Due to these teachings, to Applicants' knowledge, such spinels have not been produced nor recommended for use in the petrochemical industry.
Canadian Patent 1,028,601 issued Mar. 28, 1978 to Bagnoli et al., assigned to Exxon Research and Engineering Company discloses a high nickel (e.g. 36-38 weight %) high chromium (e.g. 23-27 weight %) steel, comprising from 1.25 to 2 weight % manganese, and the balance substantially iron. The surface of the steel may be oxidized in steam at temperatures in the range from 500° F. (160° C.) to about 2000° F. (1093.3° C.). The patent teaches that there is a protective coating of manganese and chromium oxide (chromic oxide or chromia Cr2O3) formed on the interior of the pipe. The reference teaches away from the formation of a chromium-manganese spinel (MnCr2O4). Further, the reference fails to teach the formation of oxides of manganese and/or silica selected from the group consisting of MnO, MnSiO3, Mn2SiO4 and mixtures thereof nor their use as an outer coating of a composite surface.
The abstract of Japanese Patent 57019179B teaches a ferric stainless steel comprising 16-19 weight % of Cr, 0.75-1.25 weight % of Mo, optionally copper and carbon≦0.12 weight %, Ni≦0.013, Si and Mn≦0.1, S≦0.01 weight %; is coated with a thin film of Cr2O3 having a thickness greater than or equal to 50 nm containing MnCr2O4 with or without MnSiO3. The abstract fails to teach the composite films of the present invention having a surface coating of oxides of Mn and/or Si selected from the group consisting of MnO, MnSiO3, Mn2SiO4 and mixtures thereof. The resulting surfaces have improved resistance to corrosion. The reference is silent about the resistance of the surface to coking.
U.S. Patent 5,630,887 issued May 20, 1997 to Benum et al. (assigned to Novacor Chemicals Ltd. (now NOVA Chemicals Corporation)) teaches the treatment of stainless steel to produce a surface layer having a total thickness from about 20 to 45 microns, comprising from 15 to 25 weight % of manganese and from about 60 to 75 weight % of chromium. Clearly the patent requires the presence of both manganese and chromium in the surface layer but does not teach a spinel nor oxides of Mn (e.g. MnO) and/or oxides of manganese and silicon (e.g. MnSiO3 and Mn2SiO4).
U.S. Patent 6,436,202 B1 issued Aug. 20, 2002 to Benum et al. (assigned to NOVA Chemicals (International) S.A.) and WO 02/22910; WO 02/22908 and WO 02/22905 teach a high chrome steel treated in an oxidizing atmosphere to produce a surface predominantly of a spinel of the formula MnvCr3−xO4 wherein x is from 0.5 to 2. The reference fails to teach a surface which further comprises Mn, Si, oxides selected from the group consisting of MnO, MnSiO3, Mn2SiO4 and mixtures thereof.
The present invention seeks to provide a novel surface structure having good resistance to coking.